Battery Power & Charging

An obvious thought is that a larger battery requires a larger charge. First however it is important to note that apart from common usage applied to AA and AAA batteries, for example, the word means a collection of units working together, as in a battery of guns.

Battery Cells: Nominal Voltage
Electrically chargeable cells that are used to form batteries range from a nominal voltage of between 1.2 and 2 volts. AA and AAA are 1.5v, Lithium Ion and Lead Acid are 2v. The common Lead Acid battery, used to start a car, is 12v; comprising six 2 volt cells connected in series (end to end).

Power Available: Amp Hours x Volts
Batteries have an amp hour rating which is shown on each battery be it single or a collection of cells. The label may show a value such as [48Ah] in the case of a car battery or [1500mAh](1.5Ah) on a rechargeable Lithium Ion. This figure is a measure of how much charge the battery can release over a given period of time, the most common period is 20 hours called the [20 hour rate]. And unless a battery displays an alternative rate it is assumed it is the 20 hour. So a 48Ah 12v battery has ten times more power than a 48Ah 1.2v.

The power in a battery is roughly Ah x V and in a 48Ah 12v battery is 576 watts or 0.576Kw. In comparison to mains electricity that is enough power to run a one kilowatt single bar electric fire for just over half an hour or a 50w laptop for just under 12 hours. However power output is not linear to load applied, as the quicker a battery is discharged the less efficient it is, hence the 20 hour rate parameter.

The 20 hour rate means that it will produce 48Ah at 12v only if the current drawn is constant and by calculation, at a nominal voltage of 12 that would 4 (48/12). It would power a 48 watt (12v x 4a) load for 12 hours and whereas a lighter load of 24w would run for over 24 hours a larger load of 96 watts would not run for 6 hours. The differences in the power obtained from the 20 hour rate in the examples above are not likely to be huge but the rates of discharge can be much greater. For instance some times I may use only 1 watt when running a single LED for reading but 400 watts when using an electric drill.

The lower the rate of discharge the more efficient the battery is and the less damage that occurs in a complete cycle of charge and discharge thereby extending the number of times that a battery can be recharged, varying from a few hundred to a few thousand.